1. FIeld of the Invention
The present invention relates to the field of battery charging in general, and more particularly, it relates to a multi-mode controlled current battery charging apparatus which delivers a constant current to a battery to be charged for a predetermined time. After termination of the constant current charge, the charging apparatus of the present invention provides a pulsed current to the battery to prevent self-discharge. In a preferred form of the invention, the battery charging apparatus is responsive to the temperature of the battery being charged, so that a lower constant current is delivered to the battery when the temperature of the battery increases beyond a predetermined point. The invention has particular application to Nickel-Cadmium rechargeable batteries (NiCd).
2. Description of the Prior Art
The traditional approach to charging NiCd type batteries has been to monitor the battery cell voltage and to terminate the charge once the cell has reached a predetermined voltage. This procedure works well for a single cell. With multi-cell batteries, this procedure does not succeed because the individual cells often have different voltage terminating charging characteristics. This will allow some cells to overcharge and heat up compared to others, thus damaging the overcharged cells. Additionally, when charging using the prior art charging apparatus is terminated, NiCd batteries will tend to self discharge.
Other known methods have applied constant currents to the batteries being charged, sometimes with high and low or trickle rates. Applying constant currents to NiCd batteries for an indefinite period of time causes excessive heating and gas build-up which weakens the electrolyte and greatly accelerates the decomposition of organic materials within the cell, including plate separators and seals.
Accordingly, it is an object of the invention to provide a battery charger which will charge batteries without damage to the battery.
It is a further object of the invention to provide such a battery charger which has particular application to NiCd batteries.
It is another object of the invention to provide a battery charger which prevents self discharge of batteries after they have been fully charged.
It is a still further object of the invention to provide a battery charger which prevents self-discharge of NiCd batteries after they have been fully charged by pulsing the batteries at relatively long intervals with charging current.
It is a yet further object of the present invention to provide a multi-mode constant current source battery charger which responds to the temperature of the battery being charged so as to initiate charging at a lower current rate when the battery temperature exceeds a specified temperature.
It is yet another object of the invention to provide a battery charger which can safely and efficiently charge a wide variety of batteries by accommodating a large range of cell combinations.